The HIV epidemic among male sex workers (MSW) in Thailand is understudied and incidence was estimated to have increased over the past few years. Furthermore, most MSW are undocumented ethnic minorities, are impoverished, and have low levels of education; making them particularly vulnerable to HIV infections. MSW are also highly mobile; they work in major tourist cities for few months, earn money, and then leave; thus making it extremely difficult to accurately measure HIV and STI prevalence and incidence. My hypothesis is that the characteristics of MSW are markedly different from female sex workers (FSW) in Chiang Mai, and that HIV and STI incidence is many times higher among MSW than FSW or the general population. In collaboration with the Thai Ministry of Public Health (MOPH), the proposed research will be an open prospective cohort study of male and female sex workers in Chiang Mai, Thailand. The primary goal of the < study is to describe and compare the epidemiology of HIV and STI among male and female sex workers in Chiang Mai, particularly recent versus longer-term HIV infections by use of the avidity index calculation. The Secondary goal is to determine HIV and STI social and behavioral risk factors among this population. Study findings will aid in forming targeted and culturally sensitive public health interventions for MSW in Thailand, and inform policy decisions; particularly budget allocations to this vulnerable population. Moreover, testing the feasibility of using the avidity index calculation to estimate incidence in this highly mobile population will contribute to a cost-benefit approach to HIV and STI surveillance in resource-limited settings. [unreadable] [unreadable]